Biologists are disputing the old theory that fish gills evolved to help them
breathe.

Canadian researchers, who studied gill development in rainbow trout larvae, discovered that they evolved to regulate chemicals in their bodies.

The study found that as larvae matured, fish gills regulated the chemicals in their blood before they took in oxygen.

Scientists measured the uptake of ions, such as sodium which are necessary for the body's cells to function. If levels in the blood are too high they can become toxic.

Clarice Fu, a zoologist from the University of British Columbia, who led the study, published in the Royal Society's journal Proceedings B, said fish took up these ions from water to "maintain this delicate ion balance in their blood".

"In freshwater fish, like rainbow trout, they tend to lose ions from their blood to the water, because the ion concentration in blood is greater than that of freshwater," she told the BBC.